Chemokines are a family of cytokines that regulate the adhesion and transendothelial migration of leukocytes during an immune or inflammatory reaction (Mackay C. R., Nat. Immunol., (2001) 2:95; Olson et al., Am. J. Physiol. Regul. Integr. Comp. Physiol., (2002) 283:R7). Chemokines also regulate T cells and B cells trafficking and homing, and contribute to the development of lymphopoietic and hematopoietic systems (Ajuebor et al., Biochem. Pharmacol., (2002) 63:1191). Approximately 50 chemokines have been identified in humans. They can be classified into 4 subfamilies, i.e., CXC, CX3C, CC, and C chemokines, based on the positions of the conserved cysteine residues at the N-terminal (Onuffer et al., Trends Pharmacol Sci., (2002) 23:459). The biological functions of chemokines are mediated by their binding and activation of G protein-coupled receptors (GPCRs) on the cell surface. Take CXCR4 receptor for example, it can be activated by Stromal-derived factor-1 or SDF-1, a member of CXC chemokines.
SDF-1 was originally cloned from bone marrow stromal cell lines and found to act as a growth factor for progenitor B cells (Nishikawa et al., Eur. J. Immunol., (1988) 18:1767). SDF-1 also induces bone marrow colonization of hematopoietic precursor cells during embryogenesis (Bleul et al., J. Exp. Med., (1996) 184:1101). The physiological function of SDF-1 is mediated by CXCR4 receptor. Mice lacking SDF-1 or CXCR4 receptor show lethal abnormality in bone marrow myelopoiesis, B cell lymphopoiesis, and cerebellar development (Nagasawa et al., Nature, (1996) 382:635; Ma et al., Proc. Natl. Acad. Sci., (1998) 95:9448; Zou et al., Nature (1998) 393:595; Lu et al., Proc. Natl. Acad. Sci. (2002) 99:7090). CXCR4 receptor is expressed broadly in a variety of tissues, particularly in immune and central nervous systems, and has been described as the major co-receptor for HIV-1/2 on T lymphocytes. Although initial interest in CXCR4 antagonism focused on its potential application to AIDS treatment (Bleul et al., Nature (1996) 382:829), it is now becoming clear that CXCR4 receptor and SDF-1 are also involved in other pathological conditions such as rheumatoid arthritis, asthma, and tumor metastases (Buckley et al., J. Immunol., (2000) 165:3423). CXCR4 receptor and SDF-1 are also found widely expressed in many tissues during embryonic development. Further, the CXCR4/SDF-1 pathway has been shown to be critically involved in the regeneration of several tissue injury models. Specifically, it has been found that the SDF-1 level is elevated at an injured site and CXCR4-positive cells actively participate in the tissue regenerating process.